The invention relates to a method and apparatus for non-invasively determining the inadvertent entry of a solution (local anesthetic) through a needle or catheter (conduit) into a blood vessel, and more particularly, to the use of a marker dye which is mixed with the solution and paired with a specific light emitting diode (LED) to allow rapid detection of the substance's appearance in the bloodstream by the detection of an indicator dye.
Anesthesiologists are often called upon to place conduits within the epidural space of patients in order to administer local anesthetics. These medications act directly on nerves as they enter the spinal cord, and render patients insensate to pain. Immediately prior to surgical incisions, these conduits are placed, and the patient is unable to feel the pain from surgical intervention. The technique can also be used in conjunction with general anesthesia. The conduit can be placed before the patient is anesthetized, then local anesthetics are injected during the surgical procedure. This combination of techniques, general and epidural, allows the anesthesiologist to use less general anesthesia than would be used if no conduit were employed. The conduit provides an additional benefit in post operative pain relief. The conduit is left in place with its tip in the epidural space and narcotics and/or local anesthetics are injected/infused to maintain the patient in a pain free state while recovering.
Epidural conduits are often used to care for pregnant women who are experiencing labor pains. The flexible conduit is placed into the appropriate space in a patient's back with its proximal aspect taped to the skin of the back. A continuous infusion of local anesthetic and/or narcotic acts on the nerves as they enter the spinal cord to dull the pain of uterine contraction during labor. If the solution is dilute enough, sensation is decreased while motor strength maintained. The patient is able to assist in the process of labor without the intense pain normally experienced during childbirth.
A problem with the use of this technique is the difficulty in confirming and maintaining correct conduit placement for delivery of the solution to be infused. The epidural space is located deep in the back and is surrounded by many vascular structures. If a conduit tip is accidently placed or is correctly placed but subsequently migrates into a blood vessel, the solution will be injected/infused not into the epidural space, but directly into the bloodstream. If blood levels of local anesthetic rise quickly, the patient will rapidly experience cardiovascular and neurologic complications. This is a very dangerous complication of an otherwise safe technique. The rapid administration of commonly used solutions can cause complications leading to severe morbidity, and, on occasion, mortality.
Many attempts have been made to improve the safety of the technique by mixing anesthetic solutions with other substances and medications which will provide a measurable change that can be detected by the anesthesiologist if the mixture is injected into a blood vessel. These changes can take the form of blood pressure or heart rate increases, detection of air within the vascular space, or an acute change in the patient's mental status. These substances are added in an attempt to detect the intravascular injection of local anesthetic rapidly so the injection can be stopped before medical complications develop. Yet none of the currently used substances and medications are completely reliable. Investigators have used neosynephrine, isoproterenol, chloroprocaine, microbubble preparations, and epinephrine. Epinephrine (epi) is most commonly used as an indicator. It has been used for this purpose since 1981, but it is far from ideal. Epi will not reliably cause a cardiovascular change recognized by the physician. Some people don't react as reliably as do others and the blood pressure or heart rate changes can be so small that they aren't detected by the clinician. Even very small doses of epi injection into the bloodstream have the potential to cause medical complications: hypertension, tachycardia, or bradycardia. At times, these changes are sufficient to cause ischemic changes of the heart muscle leading to arrhythmias or myocardial infarction (heart attack). Epi has also been shown to decrease blood flow from the pregnant woman's heart to her uterus. This change will decrease the flow of oxygen and nutrients to the fetus and, in some instances, put the fetus in jeopardy. Epi has also been shown to give unreliable findings in geriatric patients and in those who are concurrently receiving certain classes of medications, i.e., beta blockers for underlying medical conditions. Microbubble preparations have been made by injecting a small dose of room air into the proximal port on the epidural conduit. If the tip of the conduit is correctly placed in the epidural space, the air is harmlessly absorbed. If the tip is located within a blood vessel, the air is taken up by the blood stream and taken back to the heart. When the air bubble reaches the right atrium and right ventricle, it makes a frothy mixture of air and blood. As the heart continues to beat, this frothy mixture makes a characteristic sound that can be heard if a stethoscope or doppler measuring device is applied to the chest wall. The technique of injecting air into the conduit gives a reliable indication when the conduit tip is within a vessel but also introduces a danger. Although the frothy mixture is usually absorbed by the body with no complications, in some instances, the air can travel through an opening in the right atrium into the left atrium. In these instances, the air travels to the left ventricle and directly to the systemic circulation. If the air happens to travel to the brain, it will displace the blood that should be there and cause a cerebrovascular accident, i.e., a stroke. In addition, the use of air before the use of local anesthetic within the catheter doesn't exclude the possibility that the conduit will migrate into a blood vessel after the injection of air. There is no way to have the air act as a marker for every small amount of local anesthetic that is injected. With each heart beat, the pulsatile force is conducted to vessels in the back. The conduits which are placed in this location are affected by the movement such that the tip of the conduit can erode through the vessel wall and migrate into a blood vessel. An air test showing that the conduit tip is not in a blood vessel may be followed immediately by the injection of local anesthetic into a blood vessel.
A more reliable method is needed in which an immediate warning will occur at anytime during the course of an injection/infusion through the conduit which will alert care givers. This warning will allow corrective action to be taken before local anesthetic levels rise to high levels and cause medical complications.
It is therefore an object of the present invention to provide a means for rapidly and non-invasively determining whether a small dose of the
solution (local anesthetic) has entered a blood vessel.
It is yet another object of the present invention to provide a simple and accurate method for determining whether a conduit to be placed in the epidural space or by another collection of nerves or a nerve has entered a neighboring blood vessel.
It is yet another object of the present invention to provide a blood marker dye which is mixed with a substance such as a local anesthetic and injected near a nerve or a collection of nerves as, for example, into the epidural space, and the presence of this dye monitored non-invasively at a distant site on the patient for rapidly determining improper conduit tip placement by entry of the solution and indicator dye into the blood stream.